Generally, a door closer is an automatic door closing device that is installed on a large-sized steel door like a fire-proof door and is automatically returned slowly to its original position after the door is open.
Conventional automatic door closers have various structures in accordance with the kinds of door. Among them, there has been proposed Korean Patent No. 446221 issued to the same applicant as the present invention, which discloses an automatic door closer having a compact volume capable of improving the merchantability and mechanical durability thereof.
By the way, the conventional automatic door closers are configured wherein so as to control a door closing speed, a hydraulic circuit for adjusting an amount of oil and a hydraulic circuit of a check valve are operated cooperatively to each other, thereby making it difficult to perform the machining and raising the cost of the product. Moreover, the two hydraulic circuits are provided in the state of being not isolated from each other and have influences on each other, thereby decreasing the stability of the operation of the product.
Besides, in the conventional door closers, a shaft is disposed pierced through an outside housing and an adjuster is disposed on the side thereof, so that if fire occurs, the sealing is damaged by the external heat to cause the charged oil to leak to the outside and further to make the fire transferred to the back surface of a fire-proof door through the leaking oil.